Voltage sags and swells on distribution power systems present problems to both industrial and residential consumers. The difficulties encountered by industrial consumers of electrical power tend to be manifested as increased costs. These increased costs typically arise from down-time of process control equipment. This down-time is commonly attributable to the tripping of protective equipment. Common examples of industrial process control equipment which protects itself by tripping-off in the presence of a line overvoltage or undervoltage are the induction motor or DC motor drive. These drives control a wide range of commercial processing machinery. The cost of even a short-duration protective trip is significant.
Residential consumers, like industrial consumers, observe problems due to line swells and sags. However, the residential customer is more likely to be concerned with the perception of the power quality than the cost of the power, since short sags or swells typically do not result in a significant cost increase for a residential consumer. The most common indication of poor power quality is noticeable voltage flicker, which can be observed by the naked eye if voltage sags occur at certain rates. In addition, with the increased use of intelligent home appliances such as programmable thermostats, microwave ovens, video cassette recorders, and personal computers, both sags and swells can more easily be noticed in the home since they can cause these devices to malfunction.
The solutions to voltage variations on distribution lines have traditionally focused on voltage regulation by mechanical tap-changing transformers or mechanically switched capacitors and inductors. While both of these methods make it possible to adjust the distribution voltage to a desired level, the speed at which this is done is often unacceptable with the loads encountered on present power systems. Also, due to wear of mechanical devices, a restrictive limit is placed on the number of switching operations which can occur throughout the life of such a regulator. Therefore, rather than correcting the voltage variations due to rapidly changing loads every cycle, these regulators are usually operated only a few times a day, at most, based on expected loads at various times during the day.
Since many loads on a distribution system cause voltage variations on a cycle by cycle basis, and since loads are increasingly susceptible to malfunction because of short-duration sags and swells, a preferred method of voltage control is to apply the required correction within a cycle of the voltage irregularity. On a 60 Hertz system, as used in the United States, the possibility exists for switching 60 times per second, which precludes the use of mechanical switching devices. Even for applications that do not require cycle-by-cycle correction, there is a growing recognition of the benefit of power conditioning using rapid switching of solid state devices.
Presently, there are static VAR compensators (SVCs) in operation which utilize computers to process line voltage data and which use solid state switches to switch compensating capacitors onto the power line to provide reactive power compensation. The solid state switches must be fired at a specific time in each cycle in order to achieve transient-free switching of the capacitors onto the power line. For correctly timed switching to occur, the firing system must be synchronized with the power line fundamental frequency. In order for this synchronization to be accomplished, the method known in the art is to determine, by direct measurement with the use of circuitry, the zero-crossings of the fundamental frequency of the line voltage or current. The problem with this widely-used technique is that an unambiguous determination of the zero crossing point is difficult when system harmonics and resonances are present. In such a case, more than one zero-crossing may occur during each cycle of the fundamental frequency.
The presence of line harmonics is growing with the increasing use of solid state power conversion equipment. The harmonic problem is especially troublesome for single-phase AC circuits, because the information available for determining the zero crossings in a three-phase power system is not available in a single-phase system.
Hardware filters can be employed to reduce the measured harmonic content in a power line signal. However, hardware filters introduce waveform lag into the control system. This lag is proportional to the amount of harmonic content which must be filtered. Thus, the response time of the firing system may become limited in systems where significant harmonics are present. Attempting to reduce the lag in the hardware filter will cause the detection of multiple zero crossings and could cause a firing of the capacitor switch at the wrong point, with attendant undesirable transients or power circuit damage.
Thus, it would be highly desirable to develop a system for accurately determining the proper instant at which to activate the switches of an SVC, a capacitor bank, or other application of fast solid state switches. The switch firing system should not be sensitive to line harmonics and should not rely upon hardware filters. In addition, it would be highly desirable to develop a solid state switch firing system that does not rely upon external synchronization signals to identify the fundamental of the line signal. Finally, it would be highly desirable to develop a solid state switch firing system that can operate on a single-phase system.
The firing system of a static VAR compensator is activated in accordance with a power conditioning control strategy. Conventional power conditioning control strategies rely upon the use of a circuit equation to calculate the amount of reactive compensation required for a line. The problem with this approach is that the power system source impedance must be known. This information is difficult to secure accurately since the source impedance on an individual power system can very dynamically as loads upstream of the regulator vary. Moreover, this quantity can vary from one power system to another.
Thus, it would be highly desirable to provide a more general control strategy which can be used on a wide variety of power systems without prior knowledge of each system's specific parameters. In addition, it would be highly desirable to provide a control strategy that is rapidly executed with low computational requirements.